underpainting

例句：

1. 1. The artist's underpainting is a masterpiece in itself.
   艺术家的底层绘画本身就是一件杰作。
2. 2. The underpainting was done with great care andattention to detail.
   底层绘画非常仔细，注重细节。
3. 3. The underpainting served asa preliminary sketch for the final painting.
   底层绘画作为最终绘画的初步草图。